1. Field of Invention
The field of the currently claimed embodiments of this invention relates to terahertz ellipsometry systems, and more particularly to terahertz time-domain spectroscopic ellipsometry systems.
2. Discussion of Related Art
Terahertz time-domain spectroscopy (THz-TDS) has tremendously grown with a wide range of applications [1]-[4]. It is now relatively routine to obtain complex (i.e. real and imaginary) spectral information, for instance the complex dielectric function, with absolute numerical values when performing measurements in a transmission geometry. However, transmission measurements are not possible on many materials and so far the technique has been difficult to apply to many metals, thick or highly doped semiconductors, coatings on thick substrates, substances in aqueous solution, and any otherwise opaque compound. Moreover, an outstanding technical problem with THz-TDS continues to be the determination of absolute spectral values when performing measurements in a reflection geometry [5]-[6]. A reason why THz time-domain reflection measurements are challenging is that the time-domain technique rests on the ability to detect the relative amplitude and phase of a time-dependent electric field of a sample as compared to a reference. In transmission measurements, transmission through an aperture is used as a reference. For reflection based THz time-domain a simple mirror cannot easily be used as a reference because its surface would need to be positioned within a fraction of a micron in exactly the same place as the sample so that the reflected positioning is challenging. Ellipsometry is a well-established technique in the optical range whereby the measurement of the two orthogonal polarization components of light reflected at glancing incidence allows a complete characterization of a sample's optical properties at a particular frequency [7]. Typically, one measures the two orthogonal polarization components by a complete 360° characterization of the waves amplitude using rotating polarizers. Importantly, ellipsometry obviates the need for measurement against a standard reference sample, and so can provide reliable spectroscopic information even when surface morphology is unknown, of marginal quality and/or a reference is unavailable. It is also self-referencing, so signal to noise ratios can be very good, as source fluctuations are divided out. In order to overcome the technical problems mentioned above for THz reflectivity, ellipsometry techniques have been recently revisited for terahertz range by using a backward wave oscillator source and a Golay cell power detector [8]. There have been a number of attempts to extend ellipsometry to far-infrared frequencies using conventional Fourier Transform Spectroscopy technology [9]-[13]. Unfortunately the lack of sufficiently intense sources (in addition to the calibration issues we confront here) has meant that such efforts have been challenging, although synchrotron-based efforts have made some important contributions in this regard [12]-[13]. Generally, these studies are limited to even higher frequencies (>4 THz) than we are here.
Combining ellipsometry with THz-TDS leads to a new technique called terahertz time-domain spectroscopic ellipsometry (THz-TDSE), in which a (sub)picosecond pulse with known polarization state is used as a probe to illuminate the sample, and then the modified polarization state by the sample is detected upon reflection or transmission. Unlike conventional optical ellipsometry, the reflected (transmitted) signal is detected coherently in the time-domain which allows one to obtain both amplitude and phase of the light in the two orthogonal directions. By transforming the time-domain data into the frequency domain through Fourier analysis, it is possible to extract ellipsometric parameter spectra similar to the standard optical spectroscopic ellipsometry. However, it should be noted that the instrumentation, signal analysis and calibration methods in THz-TDSE would differ from those in the standard optical ellipsometry, and all need to be revised accordingly. M. Hangyo and his coworkers, in their pioneering works, have demonstrated the potential of THz-TDSE for measuring the complex optical constants of a Si wafer with low resistivity, the soft-mode dispersion of SrTiO3 bulk single crystals and the dielectric constants of doped GaAs thin films [14]-[16]. There are very few other reports on using the THz-TDSE [17] technique, however, there are various reports on THz polarimetry [18]-[22] for material characterization. None of the proposed THz-TDSE experimental setups can provide an easy way of changing the angle of incidence without tedious work of optical/terahertz realignments. Moreover, less attention has been paid on the compensation of the non-idealities of the optical components and the alignments in THz-TDSE through a calibration scheme. Therefore, there remains a need for improved terahertz time-domain spectroscopic ellipsometry systems.